2,5-(Hydroxymethyl)furaldehyde, also known as 2,5-(hydroxymethyl)furfural (HMF), has many important industrial and commercial applications, largely due to its many functional groups and ability to serve as a precursor in many polymerization reactions. HMF, for example, is a suitable starting source for the formation of various furan monomers required for the preparation of non-petroleum-derived polymeric materials. HMF, as well as other 2,5-disubstituted furanic derivatives, also has great potential for use in the field of intermediate chemicals from regrowing resources. Also due to its various functionalities, HMF may be used to produce a wide range of products, including, but not limited to, polymers, solvents, surfactants, pharmaceuticals, and plant protecting agents. HMF is shown in the structure below:

The use of HMF and other furfural derivatives may be compared with the use of corresponding benzene-based macromolecular compounds. In order to be cost-effective and compete in this market, HMF must be able to be produced at competitive prices. The production of HMF has been studied for years, but an efficient and cost-effective method of producing HMF in high yields has yet to be found. HMF is primarily produced from the dehydration reaction of a carbohydrate compound, particularly monosaccharides, including glucose and fructose. Complications arise from the rehydration of HMF after the dehydration occurs, which often yields the by-products of levulinic acid, and formic acid. Another competing side reaction is the polymerization of HMF and/or fructose to form humin polymers.
Hexoses are the preferred carbohydrate source from which HMF is formed. Fructose is the preferred hexose used for the dehydration reaction to form HMF. This is in part because fructose has been shown to be more amendable to the dehydration reaction to a form HMF. Fructose is shown by the structures below:

Fructose however, is more expensive than other hexoses, such as glucose (dextrose), and maltose, for example. Early processes and procedures for the production of HMF concentrated on the use of crystalline fructose, but its widespread use is prevented by its high cost. Other sources of fructose, including high-fructose corn syrup (HFCS), have been used to produce HMF and other furan derivatives. Szmant and Chundury used high fructose corn syrup as a starting material in forming HMF, as disclosed in a 1981 article in J. Chem. Tech. Biotechnol., 31, (pgs. 135-145). Szmant uses a variety of carbohydrates as starting material, but designs reaction conditions specific to each fructose source. Szmant, for example, uses a boron trifluoride catalyst (BF3Et2O) with DMSO as a solvent in the conversion of HFCS to HMF, but utilizes different catalyst/solvent combinations with different starting materials. Use of BF3Et2O as a catalyst is not economically practical since it cannot be recovered and re-used. Furthermore, Szmant requires the use of a Pluronic emulsifier to suppress foaming. Szmant also requires bubbling of nitrogen to suppress oxidation. Still further, Szmant requires the use of DMSO as a solvent, which is not easily separable from the HMF product, and therefore creates difficulties with product recovery. It is very desirable, therefore, to develop an industrially practicable process for producing HMF in high purity.
U.S. Pat. No. 6,706,900 to Grushin et al. (Grushin '900) also discloses the dehydration of fructose in the form of high-fructose corn syrup, to form HMF as an intermediate; but this process is performed in the context of forming diformylfuran, also known as 2,5-dicarboxaldehyde (DFF). The reaction proceeds in an aqueous environment, and the HMF that is formed is not isolated from the reaction mixture, but rather is directly converted to DFF without an isolation step. The reaction conditions of Grushin '900 are therefore not constrained by considerations of product yields of HMF, as it is formed as an intermediate that is not isolated as a product. More importantly from a practical commercial standpoint, Grushin '900 is not constrained by considerations of isolating HMF from the product mixture. An efficient method for producing HMF in desirable yields and sufficiently high purity from a natural and industrially convenient fructose source that may include other mixed carbohydrates has yet to be found.
Water has in the past been used as a solvent of choice in dehydration reactions forming HMF because of the solubility of fructose in water. Aqueous conditions, however, have proven to deleteriously affect the dehydration reaction of fructose to HMF in a variety of ways. Aqueous conditions have led to decreased yield of HMF as low selectivity for the dehydration reaction has been demonstrated. Furthermore, solvation of protons in water highly reduces the catalytic activity for the dehydration reaction. Low selectivity of the dehydration reaction simultaneously leads to increased polymerization reactions and humin formation, which also interfere with the synthesis of HMF.
In an attempt to solve such problems associated with aqueous systems, one proposed solution involves an improvement by simultaneously extracting HMF after the dehydration reaction. A similar attempt to improve yields involves the adsorption of HMF on activated carbon. The key factor in these processes is a rapid removal of HMF from the acidic medium in which it is formed. However, these systems generally suffer from high dilution or partially irreversible adsorption of HMF.
In another attempt to solve the problems of aqueous systems, an organic solvent may be added to the aqueous solution, such as, for example, butanol or dioxane. Such systems, however, present a difficulty in that rehydration of HMF is common and ether formation of HMF occurs with the solvent if alcohols are employed. High yields of HMF, therefore, were not found with the addition of these organic solvents. In a further attempt to provide an adequate solvent system, aqueous solvent mixtures and anhydrous organic solvents have also been employed to ensure favorable reaction conditions. Examples of anhydrous organic solvents used include dimethylformamide, acetonitrile, dimethylsulfoxide, and polyethylene glycol.
Dimethylsulfoxide (DMSO), for example, has been extensively studied and employed as a solvent in the dehydration reaction to form HMF. Improved yields of HMF have been reached with ion exchangers or boron trifluoride etherate as a catalyst, and even without any catalyst. DMSO presents a problem, however, in that recovery of HMF from the solvent is difficult.
Furthermore, although dehydration reactions performed in solvents with high boiling points, such as dimethylsulfoxide and dimethylformamide, have produced improved yields, the use of such solvents is cost-prohibitive, and additionally poses significant health and environmental risks in their use. Still further, purification of the product via distillation has not proven effective for a variety of reasons. First of all, on long exposure to temperatures at which the desired product can be distilled, HMF and impurities associated with the synthetic mixture tend to be unstable and form tarry degradation products. Because of this heat instability, a falling film vacuum still must be used. Even in use with such an apparatus however, resinous solids form on the heating surface causing a stalling in the rotor, and the frequent shutdown resulting therefrom makes the operation inefficient.
Catalysts may also be used to promote the dehydration reaction. Some commonly used catalysts include cheap inorganic acids, such as H2SO4, H3PO4, HCl, and organic acids such as oxalic acid, levulinic acid, and p-toluene sulfonic acid. These acid catalysts are utilized in dissolved form, and as a result pose significant difficulties in their regeneration and reuse, and in their disposal. In order to avoid these problems, solid sulfonic acid catalysts have also been used. Solid acid resins, however, are limited in use by the formation of deactivating humin polymers on their surfaces under conditions taught by others. Other catalysts, such as boron trifluoride etherate, can also be used. Metals, such as Zn, Al, Cr, Ti, Th, Zr, and V can be used as ions, salts, or complexes as catalysts. Such use has not brought improved results, however, as yields of HMF have continued to be low. Ion exchange catalysts have also been used, but have also delivered low HMF yields under conditions taught by others, and further limit the reaction temperature to under 130° C.